The objective of this research is to characterize the in vitro and in vivo physiology of neurons derived from embryonic stem (ES) cells. These cells represent a potentially limitless source of pluripotent, genetically normal cells for research and therapy. Mouse ES cells can differentiate in vitro into a variety of somatic cell types including neurons, astrocytes and oligodendrocytes. In addition, differentiated ES cells survive and become morphologically integrated with surrounding host tissue following transplantation into the brain or spinal cord. Based on this work with mouse ES cells, the isolation of human ES cells has raised the possibility for novel replacement therapies in which in vitro differentiated ES cells will substitute for somatic cells lost to injury or disease. Pathologies of the nervous system that might be amenable to replacement therapy include Parkinson's disease, amyotophic lateral sclerosis, stroke, Huntington's disease, and multiple sclerosis. President Bush's decision to allow federal support for research on existing human ES lines has engendered great enthusiasm to explore the promise of stem cell-derived replacement as a new way to address these previously intractable deficits; however, much basic research remains to be done before such therapies can be achieved. Our ultimate goals are to develop procedures for efficient conversion of human ES cells into specific types of neurons and to optimize the integration of ES-derived neurons into functional networks when transplanted into a host nervous system. An essential component of functional integration is the acquisition of normal physiological properties by individual stem cell derived neurons. At this point, only limited information is available about the physiology of differentiated ES cells. Thus, the goals of this proposal are: 1) To evaluate physiological differentiation of nerve cells derived from distinct ES induction protocols. 2) To characterize the physiology of ES-derived neurons after transplantation into the brain, including rigorous tests for formation of functional synaptic connections with surrounding host neurons. Human and mouse ES cells will be used in parallel to compare their developmental potentials. All research on human ES cells will use the WA01 line in the NIH human ES cell registry.